EP0794423A1 - Gas analyzer - Google Patents

Abstract

The analyser (2) has a measurement trough (21) with a radiation source (23), a detector (31) and signal processing device (3,7). The radiation source (23) can be moved along the trough (21) via a longitudinal slit (22) and then fixed in position with a locking screw (26). Typically the radiation source (23) is a modulated infra-red radiator. An interference filter (33) is arranged between source and detector (32) the characteristics of the filter being optimised for the gas components to be analysed. The measurement trough (21) is typically made from a stainless steel or aluminium tube the cross-section of which the other components, source, filter and detector are matched to.

Description

The invention relates to a gas analyzer for the continuous determination of the concentration of a gas in a gas mixture with an energy source, a measuring cell, a radiation source, a detector and devices for signal processing. The invention further relates to a measuring cell for the photometric measurement of the concentration of a gas in a gas mixture.

Gas analysis using measuring devices that work on the principle of non-dispersive infrared spectroscopy (NDIR) has been known for a long time. The areas of application are wide-ranging and include, among other things, flue gas analysis, process measurement technology in chemical process engineering and more recently the area Indoor air measurement and climate or air quality control in buildings.

The basic structure of a gas analyzer is essentially always the same. The radiation emitted by a radiation source radiates through a measuring cell with the gas to be measured and strikes a detector. On the way through the measuring cell, the initial intensity emitted by the radiation source is weakened by absorption processes. Lambert-Beer law applies to the relationship between the gas concentration to be determined and the attenuation in intensity. The generation of a detector signal with a sufficient signal / noise ratio requires modulation of the radiation emitted by the radiator. The gas to be measured either enters the measuring cell in diffusion mode or with the help of a pump.

Measuring devices of the above type are known for example from US 5,163,332 and GB 1 398 977. No. 5,163,332 describes an NDIR single-beam gas analyzer with a measuring cell, which can be operated in diffusion mode. The measuring cell consists of a closed tube which has several discrete gas access openings distributed over the length of the tube. The gas exchange takes place via a membrane that spans the gas access openings is. The radiation source and the detector are fixed at the two ends of the tube-like measuring cell. The measurement system is comparatively complicated due to the membrane system. GB 1 398 977 also describes a single-beam infrared photometer for the measurement of gases, the lamp serving as the radiation source being clocked with the aid of an oscillator. The radiation modulated in this way with a clock frequency of a few Hz passes the gas measuring section and passes through an optical filter, which is transparent to a certain wavelength, to a radiation-sensitive detector. The measuring cuvette consists of an all-round closed tube with a reflecting inner surface. The radiation source and the detector are located at the respective ends of the tube. The gas is admitted through a small opening in the area of the optical filter or detector. The use of such a clocked light source has the advantage that small, light, inexpensive, in principle battery-operated and portable, but nevertheless powerful gas analyzers can be implemented.

However, it is disadvantageous that the known cuvettes are only suitable for a narrow measuring range because the relationship between the sample gas concentration and the output signals is not linear and the measurement becomes inaccurate with increasing concentration.

The object of the invention is to develop gas analyzers of the type mentioned above in such a way that the disadvantages mentioned are eliminated, in particular an optimal adaptation of the cell lengths to the gas concentration range to be detected is possible.

The solution is that the radiation source is slidably arranged in the measuring cell.

The gas analyzer according to the invention enables an easy-to-handle adaptation of the absorption path between the radiation source and detector to different concentration ranges of the measurement gas and thus an optimization of the measurement accuracy in the concentration range to be monitored in each case, corresponding to the logarithmic decrement in a range over 5-7 orders of magnitude.

This adjustment of the measuring range is of interest, for example, for the measurement of carbon dioxide (CO ₂ ), since the CO ₂ concentration ranges to be measured can be very different depending on the application. In the area of air conditioning and ventilation technology, CO ₂ concentrations between approx. 350ppm (outside air content) and 5000ppm (MAK value) must be monitored. In flue gas measurement technology, the CO ₂ concentrations to be measured are typically between 10 and 20% by volume. In special applications there are also CO ₂ concentrations up to 100% by volume (with less resolution requirements).

When the radiation passes through the measuring medium, the intensity of the radiation is weakened by absorption. The relationships can be described quantitatively with the help of Lambert-Beer law: ln I / I ₀ = - εcd.

The ratio of transmitted radiation intensity I and source intensity Io decreases exponentially as a function of the concentration c and the length of the measuring section (cuvette length) d. The proportionality factor ε is the extinction coefficient.

Photometric measuring methods have an optimal working point for the extinction determined by the optics and electronics of the measuring device. Already from the Lambert-Beer law given above it can be seen that the relationship is not linear and that there is an ideal working area for absorbance measurement. The gas analyzer according to the invention now makes it possible for the layer thickness d, ie the distance between the radiation source and the detector, to be varied mechanically cheaply and simply, even in the case of very different gas concentration ranges to be monitored, and the Absorbance can always be kept in the optimal working area thanks to this simple mechanical adjustment. The gas analyzer according to the invention therefore enables an application-specific optimization of the measurement accuracy for any concentration ranges.

The gas analyzer is advantageously designed in such a way that the radiation source is slidably fixed in a longitudinal slot provided in the measuring cell, for example fixed with a clamping screw. It has proven to be particularly favorable to arrange the radiation source in a holder which may be provided with a reflector and which in turn is provided with a threaded hole for the clamping screw. In order to ensure the greatest possible variation in the layer thickness, the detector is fixed at one end of the measuring cell. In addition, the longitudinal slot, in which the radiation source is slidably fixed, is as long as possible and advantageously extends over the entire length of the measuring cell. This has the further advantage that the diffusion of the gas mixture to be analyzed is simplified. The rapid gas exchange, ie the good ventilation through the longitudinal slot, leads to a short T _g0 time. The mass transfer is faster. On a pump or similar can be dispensed with. The response time is reduced.

The cuvette is advantageously a metal tube, e.g. Made of aluminum or stainless steel, which can have a diffusely reflective inner surface to improve its optical properties.

It is particularly advantageous to arrange an interference filter between the radiation source and the detector. Depending on the pass band of this filter, the gas analyzer according to the invention can be adapted for each gas to be analyzed which absorbs in the wavelength range of the radiation emitted by the radiation source.

The gas analyzer advantageously sits in an at least partially gas-permeable housing, which can have an opening on one side, which is covered, for example, with a metal fiber fleece. This creates a large inlet zone for the sample gas, with the advantage of improved diffusion and convection properties. The rapid material exchange between the measuring gas space and the cuvette thereby achieved likewise leads to short response times of the gas analyzer according to the invention when the gas is applied. A convective mass transfer as a result of heating processes in the area of the radiation source can also be superimposed on the mass transfer by diffusion.

The good retention properties of the metal fiber fleece against particles, suspended matter and settable contaminants lead to a reduced tendency towards contamination of the optics of the measuring system. The gas analyzer according to the invention can also be used under difficult operating conditions, e.g. when measuring particle-laden gas flows. In addition, the metal fiber fleece acts as a flow straightener and minimizes possible dependencies of the measurement signal on aerodynamic flow conditions in the sample gas space. The metal fiber fleece can be cleaned by backwashing.

It is advantageous that the components forming the optics of the gas analysis device according to the invention are in good thermal contact with one another. This reduces the risk of thermally related misalignments of the optics and measurement errors caused thereby.

The invention further relates to an improved measuring cell for measuring the concentration of a gas in a gas mixture in the form of an elongated tube which is slit lengthways. The measuring cuvette is preferably made of metal, for example stainless steel, and preferably has a non-reflecting reflective inner surface in order to allow diffuse reflection of the beam inside the measuring cuvette. The invention The measuring cuvette advantageously has one or more bores on its underside, with which it can be fixed in a housing.

Figur 1

eine Schaltsymbolskizze des erfindungsgemäßen Gasanalysators;

Figur 2a

eine perspektivische, teilweise schematische Darstellung des erfindungsgemäßen Gasanalysators in einem Gehäuse;

Figur 2b, 2c

den Deckel des Gehäuses aus Figur 2a von oben bzw. von unten;

Figur 3

eine Draufsicht auf eine erfindungsgemäße Meßküvette;

Figur 4

eine Schnittansicht der Meßküvette aus Figur 3;

Figur 5

einen Schnitt durch eine Halterung für eine Strahlungsquelle.

An embodiment of the present invention is described below with reference to the accompanying drawings. Show it:

Figure 1

a circuit symbol sketch of the gas analyzer according to the invention;

Figure 2a

a perspective, partially schematic representation of the gas analyzer according to the invention in a housing;

Figure 2b, 2c

the cover of the housing of Figure 2a from above or from below;

Figure 3

a plan view of a measuring cell according to the invention;

Figure 4

a sectional view of the measuring cell of Figure 3;

Figure 5

a section through a holder for a radiation source.

The circuit symbol sketch in FIG. 1 relates to an NDIR single-beam photometer in microprocessor technology. A CO ₂ sensor was chosen as the exemplary embodiment. CO ₂ has an absorption maximum at a wavelength of 4.24 µm. An infrared radiation source is therefore required as the radiation source, for example in the form of a long-life, low-drift, miniaturized IR radiator. In the simplest and preferred case, this can be a miniature light bulb. Surface emitters in thick-film and thin-film technology can also be used as an IR radiation source.

For modulation, the IR radiation source is clocked using an oscillator. When using a miniature light bulb, the clock frequency is a few Hz; the use of thin-layer emitters enables clock frequencies of up to 100 Hz. The omission of mechanically moving parts (e.g. a chopper wheel) enables a miniaturized design.

The electromagnetic radiation emitted by the IR radiation source passes through the measuring section with the gas to be measured and, after passing through an interference filter, strikes a detector. The interference filter can be used as a discrete component, but can also be integrated in the detector. Of the The pass band of the interference filter is matched to the absorption maximum of a specific absorption band of the gas component to be determined, in the example to the CO ₂ absorption band at 4.24 μm. An infrared-sensitive electronic component is used as the detector, for example a pyroelectric detector or a thermopile. Semiconductor components (PbS, PbSe) can also be used. The output signal of the radiation-sensitive detector is detected with a phase-controlled AC amplification circuit with tunable zero point detection.

The clocking of the radiation source, which is necessary, for example, when using a detector that only responds to the radiation intensity, requires a periodically delayed increase and a decay of the thermally induced emission before or after reaching the maximum lamp temperature. Capacitive or comparable electronic coupling methods allow AC voltage symmetrization of the modulated detection signal. The phase switching point for the phase-dependent signal amplification can be determined by triggering the zero point pass. During the useful signal phase, an emitter temperature optimized for the absorption of the measuring gas can be presented. Signal components during the thermal decay and The heating-up time can be compensated against the useful signal phase in order to achieve interference signal elimination. The signal output is either an electrical voltage (0 to 10 V) or a current (0/4 to 20 mA).

Together with an integrated temperature sensor circuit, the output of a temperature-compensated signal is made possible. Another possibility is to use a temperature-dependent voltage source. FIG. 2a shows a possible embodiment of the gas analyzer 20 according to the invention. The housing 2 accommodates the electronics, of which only one circuit board 3 is indicated on the bottom of the housing 2. There are threaded bores 5 in the corners 4 of the housing 2. The signal output 7 is located on a side wall 6 of the housing 2 and is connected to the circuits on the circuit board 3 via cable 8 and also serves to supply energy. 9 denotes an electrolytic capacitor 220 µF.

Figures 2b and 2c show a cover 10 for closing the housing 2. It has 11 holes 12 at its four corners. The lid 10 is placed on the housing 2 so that the bolts 5 and 12 are aligned, and is screwed.

The lid 10 also has an opening 13 which the Gas passage serves. The opening 13 is closed on its side facing the outside atmosphere with a metal net 14. On the side oriented towards the measuring cell 21, as can be seen from FIG. 2c, a metal fiber fleece 16 is fastened to the metal net 14 or the inner surface of the lid 10, for example glued on with adhesive 15. The metal fiber fleece 16 preferably consists of fibers with diameters of up to 2 μm, which are read into random fibers in uniform basis weights and rolled to defined thicknesses. Corresponding metal fiber fleeces have porosity levels of up to 80% with a very narrow pore size distribution. This creates a large-area inlet zone for the measuring gas, with the advantage of improved diffusion and convection properties and a lower tendency to contamination, which increases the service life of the gas analyzer.

It can be seen from FIGS. 2 and 3 that the measuring cuvette 21 of the gas analyzer 20 according to the invention consists of an elongated tube with a longitudinal slot 22, which is not shown somewhat too large in FIG. 3. Due to the longitudinal slot 22 oriented towards the sample gas inlet 21, the sample gas can be rapidly supplied and removed by diffusion and convection. The radiation source 23, in the present case a miniature light bulb, sits in a holder 24 and is bordered by a reflector 25. FIG. 4 again shows in detail for the exemplary embodiment that the measuring cuvette 21 has an outer diameter D _k of 10 mm and an inner diameter d _k of 8 mm with a length l _k of approximately 70 mm. The longitudinal slot 22 is about 2mm wide. On the underside of the measuring cell 21 opposite the longitudinal slot 22 there are two holes 17, 28 with an M3 thread, the hole 27 being approximately 7.5 mm from the rear end 21 'of the measuring cell 21 and the hole 26 being approximately 37 mm away. The material is V4A stainless steel.

Ideally, the measuring cell 21 with the longitudinal slot 22 is located directly behind the metal net 14 and closes tightly therewith. Then the gas volume to be flushed is almost exactly the (small) volume of the measuring cuvettes, so that short response times can be achieved. In this case, the measuring gas space is actually the entire geometric space above the metal wire network (e.g. office space, lecture hall). With 2 'is a dead space in the housing 2, which, however, must be rinsed out. This dead space 2 'should be kept small.

In Figure 5, the bracket 24 for the radiation source 23 is shown again. It has a diameter d _H of approximately 8 mm, which corresponds to the inner diameter d _{k of} the measuring cell 21 and a length l _H of 6 mm. One to one end the radiation source 23 receives conically widened through opening 29. The conical extension is designed as a reflector 25, which comprises the radiation source 23. At a right angle to the through opening 29, the holder 24 has a further bore 30 with an M2 thread for the clamping screw 26. The holder 24 is thus inserted into the measuring cell 21, the bore 30 being oriented toward the longitudinal slot 22. Then the clamping screw 26 is screwed into the bore 26. The head of the clamping screw 30 is wider than the longitudinal slot 22, so that a clamping effect between the clamping screw 26 and the measuring cell 21 is effected when screwing into the bore 30. In this way, the holder 24 with the IR radiation source 23 can be guided in the longitudinal slot 22 of the measuring cell, which is open towards the measuring gas, in the direction of arrow A in FIG. 3.

FIG. 3 also shows a detector 31 which is fixed to the end 21 'of the measuring cuvette 21 remote from the radiation source 23. The detector 31 is connected to the circuit on the circuit board 3 by lines 32. The detector 31 has the largest possible detector area, which corresponds approximately to its cross-sectional area in the exemplary embodiment. An interference filter 33 is also connected in front of the detector 31. The interference filter can also be integrated in the detector. He is in Exemplary embodiment for IR radiation in the wavelength range of 4.24 µm transmissive. The length of the measuring section, ie the distance between the radiation source 23 and the detector 31, is variable and can be optimized for different gas concentration ranges.

The individual components that form the optics of the sensor are in good thermal contact in this exemplary embodiment. In connection with the guiding of the radiation source 23 along the measuring section, the diffuse reflection on the inner wall of the measuring cell 21 and the use of a detector 31 with a large dimensioned detector surface, thermally induced misalignments of the optics and measurement errors caused thereby are negligible. This has advantages over gas sensors with complex imaging optics when using mirrors that require precise adjustments and isothermally guided mechanical assemblies.

Reference list

1

Overall device

2nd

casing

2 '

Sample gas space

3rd

circuit board

4th

Corners of 2

5

Tapped holes

6

Sidewall of 2

7

Signal output

8th

electric wire

9

battery

10th

Cover of 2

11

Corners of 10

12th

Holes

13

Opening in 10

14

Metal mesh

15

adhesive

16

Metal fiber fleece

20th

Gas analyzer

21

Measuring cell

21 '

End of 21

22

Longitudinal slot in 21

23

Radiation source

24th

Bracket for 23

25th

reflector

26

Clamping screw

27.28

Holes in 21

29

Through opening in 24

30th

Hole in 24

31

detector

32

cables

33

Interference filter

A

Direction of displacement of the light source

l _k

Length of 21

D _k

Outside diameter of 21

d _k

Inner diameter of 21

l _H

Length of 24

d _H

Diameter of 24

Claims (19)

Gas analyzer (20) for the continuous determination of the concentration of a gas in a gas mixture, with a measuring cell (21), a radiation source (23) arranged therein, a detector (31) and devices for signal processing (3, 7), characterized in that the Radiation source (23) in the measuring cell (21) is arranged displaceably. Gas analyzer according to claim 1, characterized in that the radiation source (23) is displaceably fixed in a longitudinal slot (22) provided in the measuring cell (21). Gas analyzer according to claim 2, characterized in that the radiation source (23) is fixed with a clamping screw (26). Gas analyzer according to claim 2, characterized in that the radiation source (23) is held in a holder (24) which has a bore (30) for the clamping screw (25). Gas analyzer according to claim 4, characterized in that the holder (24) has a reflector (25). Gas analyzer according to one of the preceding claims, characterized in that the detector (31) is attached to the end (21 ') of the measuring cuvette (21) remote from the radiation source (23). Gas analyzer according to one of the preceding claims, characterized in that the detector area of the detector (31) corresponds approximately to its cross-sectional area. Gas analyzer according to one of the preceding claims, characterized in that the measuring cell (21) is in the form of a metal tube, preferably made of aluminum or stainless steel. Gas analyzer according to one of claims 2 to 8, characterized in that the longitudinal slot (22) extends over the entire length of the measuring cell (21). Device according to one of the preceding claims, characterized in that the measuring cuvette (21) has a diffusely reflecting inner surface. Gas analyzer according to one of the preceding claims, characterized in that between the Radiation source (23) and the detector (31) an interference filter (33) is arranged, the pass band is matched to the absorption maximum of a specific absorption band of the gas component to be determined. Gas analyzer according to claim 11, characterized in that the interference filter (33) is integrated in the detector (31). Gas analyzer according to one of the preceding claims, characterized in that it is arranged in an at least partially gas-permeable housing (2). Gas analyzer according to claim 13, characterized in that the housing (2) has an opening (13) covered with a metal fiber fleece (16). Gas analyzer according to claim 13 or 14, characterized in that the components forming the optics are arranged in the housing (2) in such a way that they are in good thermal contact with one another. Measuring cuvette (21) for the photometric measurement of the concentration of a gas in a gas mixture, characterized in that it is in the form of a entire length is formed with a longitudinal slot (22) provided tube. Measuring cell according to claim 16, characterized in that it is made of metal, e.g. Aluminum or stainless steel. Measuring cell according to claim 16 or 17, characterized in that its inner surface is diffusely reflective. Measuring cell according to one of claims 16 to 18, characterized in that it has at least one bore (27, 28) on the side opposite the longitudinal slot (22) for fixing in a housing (2).

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